The present invention relates to a process for the production of a hydrogen peroxide solution which includes electrolyzing seawater to produce hydrogen peroxide with secondary production of effective chlorine or organic halogen compound in a minimized amount.
Due to concern that the pollution and the deterioration of water quality of rivers and lakes caused by industrial and household wastes can have adverse effects on the environment and human body, technical countermeasures for solving these problems are urgently needed. In the treatment of drinking water, sewage and waste water, the practice has been to add a chemical such as chlorine to decolor and sterilize the water to be treated and reduce the COD thereof. However, since the addition of a large amount of chlorine causes the production of harmful materials, i.e., environmental hormones (extrinsic incretion disturbing material) and carcinogenic substances, the recent trend is to add less chlorine.
Further, under some combustion conditions, the incineration of waste can cause the production of carcinogenic substances (dioxins) which affect the ecosystem and thus has been noted as a safety problem. In order to solve the problems of water treatment, the following water treatment processes have been proposed.
An example of chemicals suitable for sterilization in water treatment is hydrogen peroxide. Hydrogen peroxide is useful as a fundamental chemical indispensable for treatment in the fields of food, medicine, pulp, fiber and semiconductors in addition to water treatment. In particular, noted future uses of hydrogen peroxide include cleaning of electronic parts and sterilization of medical equipment and facilities. At present, hydrogen peroxide is produced in a large amount by an anthraquinone process.
In power plants and factories using seawater as cooling water, it has been heretofore practiced to directly electrolyze seawater to produce hypochlorous acid which is then effectively used to prevent the attachment of organisms such as barnacles and alga to the interior of the condenser.
However, when hypochlorous acid is discharged untreated, it decomposes to produce organic chlorine compounds and chlorine gas, which are considerably harmful to the environment. Stricter regulations have been imposed on the discharge of hypochlorous acid.
On the other hand, it has been reported that addition of hydrogen peroxide to cooling water effectively prevents the attachment of living organisms. Further, hydrogen peroxide decomposes to water and oxygen, which are harmless and raise no environmental and hygienic problems.
However, hydrogen peroxide is too unstable to store over an extended period of time. Also, from the standpoint of safety and anti-pollution measures, there has been a growing demand for an on-site device. An electrolysis process has been proposed as an on-site process for the production of hydrogen peroxide.
An electrolysis process can utilize clean electric energy to carry out a desired electrochemical reaction. By controlling the chemical reaction on the surface of the cathode, the electrolysis can produce hydrogen peroxide. Water treatment involving the decomposition of contaminants by this electrolysis process has long been widely practiced. This electrolysis process allows onsite production of hydrogen peroxide, eliminating the disadvantage of hydrogen peroxide with respect to poor storage stability in the absence of a stabilizer, the danger in transportation and the necessity of anti-pollution measures.
Referring to the production of hydrogen peroxide by electrolysis, various electrolytic production processes are described for comparison in Journal of Applied Electrochemistry, Vol. 25, 613-(1995). All these processes allow efficient production of hydrogen peroxide in an atmosphere of an alkaline aqueous solution and thus require the supply of an alkaline component as a starting material. Thus, an aqueous solution of an alkali such as KOH and NaOH is essential. As an example of the decomposition of an organic compound by hydrogen peroxide, the decomposition of formaldehyde is described in Journal of Electrochemical Society, Vol. 140, 1,632-(1993). Journal of Electrochemical Society, Vol. 141, 174-(1994), proposes a method which comprises electrolysis of purified water as a starting material using an ion exchange membrane wherein ozone and hydrogen peroxide are synthesized at the anode and the cathode, respectively. However, this method has a low current efficiency and thus is not practical. It has been reported that a similar method can be effected under high pressure to raise the current efficiency. However, this proposal, too, is not practical from the standpoint of safety. An electrolysis process using palladium foil has been proposed. However, this electrolysis process is limited in its use because it can produce hydrogen peroxide only in a low concentration and adds to cost.
When seawater is subjected to electrolysis as an electrolyte with oxygen present on the cathode side, the resulting hydrogen peroxide is dissolved in the seawater to produce a hydrogen peroxide solution. In this manner, the hydrogen peroxide and a superoxide anion (O2xe2x88x92) produced therewith sterilize microorganisms in the seawater to obtain a hydrogen peroxide solution having high purity. When the anode used for this electrolysis process is a commercial oxygen producing electrode, halide ions in the seawater, i.e., chloride ions in a high concentration and fluoride ions, bromide ions and iodide ions in a slight amount are oxidized at the anode to produce a halogen gas such as chlorine gas or a hypohalogenous acid such as hypochlorous acid. Even when an electrode which resists the generation of chlorine gas or the like is used or when a cation exchange membrane is used to separate the cathode from the anode, which is a site for the production of chlorine gas, the oxidation of chloride ions or the like cannot be completely prevented.
Further, chlorine gas or the like is likely to react with organic compounds in the seawater to produce a harmful trihalomethane (THM). In order to prevent the production of THM, water treatment may be effected using a hydrogen gas anode while supplying hydrogen gas (Japanese Patent Laid-Open No. 1998-121281). In this manner, the oxidation of chloride ion (i.e., production of chlorine gas or hypochlorous acid) can be inhibited, making it possible to eliminate the source of THM. However, this method requires the installation of a hydrogen gas anode and the supply of hydrogen gas, adding to cost. Thus, this method is not economical. Further, this method involves a danger in handling hydrogen gas.
It is also known that organic compounds in seawater partly undergo oxidative destruction at the anode to produce chlorine. To solve this problem, one technique proposes the use of an insoluble anode which resists the production of chlorine gas and an ion exchange membrane (Japanese Patent Laid-Open No. 1999-158674). However, because seawater containing a large amount of organic compounds is used as an anolyte, the production of THM is unavoidable.
As discussed above, when seawater containing an organic compound and halide ions is subjected to electrolysis in a conventional manner to produce seawater containing hydrogen peroxide, the production of organic halogen compounds such as THM unavoidably occurs, raising a great environmental and hygienic problem.
It is therefore an object of the invention to provide a process for the production of a hydrogen peroxide solution using a solution having a low halide ion concentration as an anolyte for the production of seawater containing hydrogen peroxide by the electrolysis of seawater. The process of the invention practically avoids the production of effective chlorine or THM, which is unavoidable in conventional processes for the production of hydrogen peroxide solution from seawater.
The above object of the present invention will become apparent from the following detailed description and Examples.
The invention provides a process for the production of hydrogen peroxide solution which comprises effecting electrolysis while supplying an anolyte, a catholyte and an oxygen-containing gas to an anode chamber having an insoluble anode, a solution chamber and a gas chamber, respectively, of a hydrogen peroxide producing electrolytic cell to produce a hydrogen peroxide solution. The hydrogen peroxide producing electrolytic cell is partitioned by a membrane into an anode chamber and a cathode chamber housing a gas diffusion cathode. The gas diffusion cathode partitions the cathode chamber into a solution chamber and a gas chamber. Furthermore, the catholyte is seawater and the concentration of halide ion in the anolyte is not greater than 1 g/l.